The DOBA is a high acid crude oil originating from the Chad region of West Africa. The DOBA is known to contain calcium naphthenate and the amount of calcium naphthenate varies over a range from about 150 to about 700 ppm. In a typically supplied DOBA crude oil, the amount of calcium naphthenate may vary from about 250-about 300 ppm.
The DOBA is a heavy high acid crude oil with Total Acid Number [TAN] ranging upwards of 4.0 mg KOH/gm of sample and the American Petroleum Institute (API) gravity is about 19. However, the sulfur content in DOBA is very low to nil.
The DOBA is typically a crude oil with a lot of residue in it and for proper blending, typically internationally, refiners blend it with very light crude oil or condensates to increase the API of the resulted blend to more than 30. Such blending with light crude oil or condensates helps to create sufficient light ends to help achieve product yields for the crude distillation unit. The most of light crude oil or condensates thus selected generally have very little to nil sulfur contents or existent and potential H2S, which means the overall sulfur content still remains very low. Further, the H2S being oil soluble is not present in relatively higher quantities in these types of blends.
The inventor has observed when DOBA or its above described blends containing calcium naphthenate, but having sulfur contents or H2S being less than 0.2% is treated with additives as known in the art, for example with glycolic acid [US patent publication No. 2009/0152164 and U.S. Pat. No. 7,497,943—(US'943)], the presence of low amounts of sulfur in DOBA crude oil or its blends does not produce impurity, and hence, does not hamper removal of metals including calcium from such DOBA crude oil or its blends.
However, if the refiner processes the DOBA or its blends having API varying between about 24 to about 29 and a crude oil having sulfur or H2S contents in higher amounts as compared to above situation, the presence of higher amounts of sulfur or H2S contents in presence of water will have first chance to react with calcium naphthenate and as a result several reactions occur, which have been found to produce insoluble and soluble impurities, wherein the insoluble impurities get accumulated in the form of a black layer at the interphase of organic and aqueous layers during isolation of metals including calcium from the DOBA or its blends. The formation of impurities due to reactions between calcium naphthenate and sulfur compounds or H2S in presence of water and their accumulation at the interphase of organic and aqueous layers has been found not only to hamper processing of crude oils, but also the removal of metals including calcium.
The inventor of present invention has observed that if solution of calcium naphthenate in an organic solvent, for example toluene having concentration of Ca of about 2247 ppm is treated with equal weight of water by heating to about 130 degree C., that is, in absence of sulfur compound or H2S in a Parr autoclave under autogenous pressure, and separated into organic and aqueous layers in a separating funnel, no black layer is formed at the interphase which indicates that calcium naphthenate does not hydrolyze in absence of sulfur compound or H2S, which also indicates that no impurities are formed which could have accumulated at the interphase. When organic layer, as separated, was dried by evaporating toluene, its acid value was found to be very low of about 48.36 (mg KOH per gm) which also indicates that calcium naphthenate did not hydrolyze meaning thereby the calcium is not removed from the oil. When IR of dried organic material was taken, as shown in accompanying FIG. 1, presence of peaks at about 1555.3 cm−1 and at about 1678.7 cm−1 also indicates that calcium naphthenate did not hydrolyze in absence of sulfur or H2S meaning thereby the calcium is not removed from the oil. In the description herein, this experiment may be referred as experiment 1.
When solution of calcium naphthenate in toluene having concentration of Ca of about 2247 ppm and saturated with H2S [or sulfur compound] by purging H2S in a manner to have concentration of H2S in the vapour phase of about 13 vol % was treated with equal weight of water by heating to about 130 degree C., that is, in presence of sulfur compound or H2S, in Parr autoclave under autogenous pressure, calcium naphthenate was found to react with H2S [or sulfur compound] in presence of water and on separation of organic and aqueous layers after cooling to room temperature in a separating funnel, a black layer was formed at the interphase which indicated that calcium naphthenate gets hydrolyzed in presence of H2S [or sulfur compound] and water, and impurities formed thereby get accumulated at the interphase. When organic layer, as separated, was dried by evaporating toluene, its acid value was found to be high of about 156.26 (mg KOH per gm) which also indicated that calcium naphthenate got hydrolyzed meaning thereby calcium is removed from the calcium naphthenate in oil or hydrocarbon. When IR of dried organic material was taken, as shown in accompanying FIG. 2, presence of weak peak at 1535.7 cm−1 and no peak at 1680.7 cm−1, but strong peak at 1696.2 cm−1 in the IR spectrum also indicated that calcium naphthenate is hydrolyzed in presence of H2S [or sulfur compound] and water meaning thereby calcium is removed from the calcium naphthenate in oil or hydrocarbon. In the description herein, this experiment may be referred as experiment 2.
When experiment 2 was repeated in a manner to have concentration of H2S in the vapour phase of about 5 vol % to see if the lower concentration of H2S also results in formation of impurities and their accumulation at the interphase, it was found that even at lower concentration of 5 vol % of H2S impurities are formed and get accumulated at the interphase even for the period up to 60 min. In the description herein, this experiment may be referred as experiment 2A. The calcium concentration in the organic layer, as measured by Inductive coupled plasma (ICP) and acid value (AV) of the compound obtained after drying the organic layer are provided against time in the Table-I, which indicate that only up to 39.9% of Ca is removed without any additive:
TABLE ISr.Acid Value% EfficiencyCa Conc.% EfficiencyNo.Time(MgKOH/gm)(by AV)(ppm)(by Ca)1 5 min60.0526.1161728215 min66.9729.6154531.2330 min77.6233.7146534.8460 min88.3338.4135039.9
Accordingly, the industry processing DOBA or its blend containing calcium naphthenate and a crude oil containing sulfur compounds or H2S faces problems of formation of impurities upon reaction of calcium naphthenate and sulfur compounds or H2S in presence of water for example when such crude oils are processed through desalter and such impurities continue to get accumulated in the system which not only hampers processing of crude oils, but also adversely effect removal of metals including calcium.